day 43: 5-longest-palindromic-substring

5-longest-palindromic-substring/driver.cpp

@@ -0,0 +1,10 @@
+#include "soln.hpp"
+
+int main()
+{
+	string s = "varorant";
+	string answer = "arora";
+	Solution soln;
+	cout << "Found longest palindromic substring correctly? " << soln.test(s, answer) << endl;
+	return 0;
+}

5-longest-palindromic-substring/soln.cpp

@@ -0,0 +1,54 @@
+#include "soln.hpp"
+
+string Solution::longestPalindrome(string s)
+{
+	int n = s.size();
+
+	// base cases
+	if (n == 0)
+		return "";
+	if (n == 1)
+		return s;
+
+	// DP memorization table indices are substring start-end pairs
+	vector<vector<bool>> dp(n, vector<bool>(n, false));
+
+	// single length strings/characters are palindromes
+	for (int i = 0; i < n; i++)
+	{
+		dp[i][i] = true;
+	}
+
+	// start from first character
+	string ans = s.substr(0, 1);
+
+	// start from last, decrease
+	for (int start = n - 1; start >= 0; start--)
+	{
+		// for each, end at last, increase
+		// so we go through the string's substrings from the end
+		for (int end = start + 1; end < n; end++)
+		{
+			if (s[start] == s[end])
+			{
+				// either two characters, or extension of existing palindrome
+				if (end - start == 1 || dp[start + 1][end - 1])
+				{
+					// mark as palindrome
+					dp[start][end] = true;
+					// check if larger
+					if (ans.size() < end - start + 1)
+					{
+						ans = s.substr(start, end - start + 1);
+					}
+				}
+			}
+		}
+	}
+	return ans;
+}
+
+bool Solution::test(string s, string answer)
+{
+	return longestPalindrome(s) == answer;
+}

5-longest-palindromic-substring/soln.hpp

@@ -0,0 +1,8 @@
+#include <bits/stdc++.h>
+using namespace std;
+class Solution
+{
+public:
+	string longestPalindrome(string s);
+	bool test(string s, string answer);
+};